1. Field of the Invention
This invention relates to improved methods for forming an alloyed conductive layer on a semiconductor body and improved methods for hermetic passivation of the conductive layer.
2. Prior Art
In the manufacture of semiconductor devices, it is generally necessary to interconnect two parts of an integrated circuit by means of a conductive layer. Often this is done by first depositing a metal layer of aluminum/silicon alloy into a "contact window", (i.e., an aperture made in the insulating layers of the semiconductors devices). Next, the metal layer is patterned by methods standard in the integrated circuit industry. Subsequently, the patterned circuit is heated or alloyed to improve the contact resistance of the metal to the silicon substrate.
One problem associated with patterning of the conductive layer is undercutting caused when light reflects off the conductive layer. Prior art attempts to solve this problem have included the use of a layer of polyimide as an anti-reflective coating (ARC). The disadvantages of this process is that it has a narrow process window and a more complicated procedure during reworking.
A second problem, associated with alloying, is that hillocks form on the surface of the resulting metal layer and voids form within the layer. The surface hillocks make subsequent masking of the layer difficult while the voids adversely affect the conductive properties of the layer.
One attempt to solve this problem uses TiWN coatings on the metal layer to suppress hillock formation. See "Reduction of Hillock Formation in Aluminum Thin Films" Semiconductor International, April 1982. Another method deposits a layer of SiO.sub.2 glass, see "Hillock Growth on Vacuum Deposited Aluminum Films" The Journal of Vacuum Science and Technology Vol. 9, No. 1. With this method it is difficult to maintain the integrity of upper level glass and metal layers. In "Surface Reconstruction of Aluminum Metallization--A new Potential Wearout Mechanism" E. Philofsky et. al., it was noted that glassing and adding alloy additives to aluminum retarded low temperature surface reconstruction of metal films (p. 122) and that problems of high current density or shorting due to surface reconstruction could be reduced or eliminated by glassing over or alloying with another element in aluminum (p. 123). The above processes have not proven satisfactory in preventing hillocks or voids.
After the conductive layer has been alloyed and patterned, it is generally sealed by hermetic passivation. Plastic packaging calls for a hermetic passivation such as plasma silicon nitride. In the prior art, a compressive film (films applying a tensile stress on the underlying conductive layer) is used to keep the surface hermetic. One resulting problem is cracking and voiding in the conductive layer. During cooling of the semiconductor device after passivation, the conductive layer contracts more rapidly than the sealing layer. That layer is, in effect, holding onto the conductive layer while the conductive layer shrinks during cooling. This puts large stresses on the conductive layer, stress of sufficient magnitude to cause cracking and metal grain migration on the surface of the conductive layer. As a result, current densities in the conductive layer reach levels beyond their design limits. A typical upper level of current density in certain devices is 5.times.10.sup.4 amps/cm.sup.2. However, due to the reduction of the cross sectional area, levels can increase tenfold in damaged conductive layers, to as high as 5.times.10.sup.5 amps/cm.sup.2, leading to unreliable performance or failure.
The present invention is directed toward reducing defects in the conductive layer in semiconductor devices.